Guide to TCP/IP, Third Edition

Guide to TCP/IP, Third Edition

Chapter 3: Data Link and Network Layer TCP/IP

Protocols

Guide to TCP/IP, Third Edtion 2

Objectives

• Understand the role that data link protocols, such as SLIP and PPP, play for TCP/IP

• Distinguish among various Ethernet and token ring frame types

• Understand how hardware addresses work in a TCP/IP environment, and the services that ARP and RARP provide for such networks

Guide to TCP/IP, Third Edtion 3

Objectives (continued)

• Appreciate the overwhelming importance of the Internet Protocol (IP), and how IP packets behave on TCP/IP networks

• Understand the lifetime of an IP datagram, and the process of fragmentation and reassembly

• Appreciate service delivery options

• Understand IP header fields and functions

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Data Link Protocols

• Key jobs of Data Link layer – Media Access Control (MAC)– Logical Link Control (LLC)

• Point-to-point data transfer– Shipping data from one MAC layer address to

another

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Data Link Protocols (continued)

• WAN encapsulation of frames at Data Link layer involves one or more of the following services– Addressing– Bit-level integrity check– Delimitation– Protocol identification (PID)

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Serial Line Internet Protocol (SLIP)

• Original point-to-point protocol

• Sometimes used to – Manage communications or networking equipment

through a dial-up serial port connection

• Simple packet-framing protocol described in RFC 1055

• Uses a special END character (0xC0)– Placed at the beginning and end of each IP

datagram to delimit, or separate, each payload

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Point-to-Point Protocol

• Provides – Frame delimitation– Protocol identification and bit-level integrity check

services• RFC 1661 includes

– Encapsulation methods – A special Link Control Protocol (LCP)– A collection of negotiation protocols

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Point-to-Point Protocol (continued)

• Fields in the PPP header and trailer include– Flag– Protocol identifier– Frame Check Sequence (FCS)

• Supports a default MTU of 1,500 bytes– Which makes it ideal for interconnecting Ethernet-

based networks (or peers)

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Special Handling for PPP Links

• For switched technologies– Bidirectional connections must be negotiated

between peers that wish to exchange data

• X.25: RFC 1356. X.25– Standard set of protocols defined in the 1970s by the

International Telecommunications Union (ITU)• Frame relay: RFC 2427

– Assumes that digital-quality transmission lines are available for creating WAN links

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Special Handling for PPP Links (continued)

• ATM: RFCs 1577 and 1626– High-speed, long-haul, broadband, cell-switched

networking technology– Offers astonishing and ever-increasing bandwidth

• PPPoE: RFC 2516 – Protocol used by Internet service providers to

authenticate and manage broadband subscribers

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Frame Types

• At Data Link layer– Protocol data units are called frames

• Frame – Represents same data that appears in digital form

at the Network layer in an IP datagram

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Ethernet Frame Types

• Ethernet II frame type – De facto standard frame type used for IP datagram

transmissions over Ethernet networks– Has protocol identification field

• Ethernet frame types that TCP/IP can use– Ethernet II– Ethernet 802.2 Logical Link Control– Ethernet 802.2 Sub-Network Access Protocol

(SNAP)

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Ethernet II Frame Structure

• Ethernet II frame type fields and structure– Preamble– Destination Address Field– Source Address Field– Type Field– Data Field– Frame Check Sequence Field

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Ethernet II Frame Structure (continued)

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Ethernet II Frame Structure (continued)

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Ethernet 802.2 LLC Frame Structure

• Unique fields– Preamble– Start Frame Delimiter Field:– Length Field– Destination Service Access Point (DSAP) Field:– Source Service Access Point (SSAP) Field:– Control Field– Destination Address– Source Address– Data– Frame Check Sequence

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Ethernet 802.2 LLC Frame Structure (continued)

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Ethernet 802.2 LLC Frame Structure (continued)

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Ethernet SNAP Frame Structure

• Fields– Organization Code Field– Ether Type Field– Preamble– Start Frame Delimiter– Destination Address– Source Address– Length– Destination Service Access Point– Source Service Access Point

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Ethernet SNAP Frame Structure (continued)

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Token Ring Frame Types

• IEEE 802.5 standard – Defines token ring networking

• Token ring networks– Rely on a physical star design, although they use a

logical ring transmission path

• On a token ring network– Each token ring workstation acts as a repeater

• Variations of token ring frames– Token Ring 802.2 LLC frames– Token Ring SNAP frames

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Token Ring Frame Types (continued)

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Hardware Addresses in the IP Environment

• IP addresses – Identify individual IP hosts on a TCP/IP internetwork

• TCP/IP networking uses ARP to– Determine the hardware address of the local target

for the packet

• ARP cache – Table of hardware addresses learned through the

ARP process

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Hardware Addresses in the IP Environment (continued)

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Hardware Addresses in the IP Environment (continued)

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Hardware Addresses in the IP Environment (continued)

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ARP Packet Fields and Functions

• Basic ARP packets– Broadcast ARP request packet– Directed, or unicast, ARP reply packet

• Most confusing part of ARP – Interpretation of the sender and target address

information

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ARP Packet Fields and Functions (continued)

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ARP Packet Fields and Functions (continued)

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ARP Cache

• ARP information – Kept in an ARP cache in memory on most operating

systems

• Windows-based systems– Command arp -a is used to view the table contents– Have utility to view IP and hardware addresses

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ARP Cache (continued)

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Proxy ARP

• Method that allows IP host to use a simplified subnetting design

• Enables a router to “ARP” in response to an IP host’s ARP broadcasts

• Most network configurations– May never need to use proxy ARP

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Proxy ARP (continued)

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Reverse ARP

• Used to obtain an IP address for an associated data link address

• Initially defined to– Enable diskless workstations to find their own IP

addresses upon booting or startup

• BOOTP, and eventually DHCP, replaced RARP

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About Internet Protocol

• Network layer communications – End-to-end communications

• Internet Protocol – Network layer protocol used in the TCP/IP suite

• IP version 4 (IPv4)– Widely implemented

• Internet Protocol version 6 (IPv6)– Most used in pilot or experimental implementations

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Sending IP Datagrams

• Requirements for building an IP datagram packet to transmit on the wire– IP addresses of the source and destination– Hardware address of the source and next-hop router

• IP host – Can use a manually entered destination IP address

or the DNS to obtain a destination’s IP address

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Route Resolution Process

• Enables IP host to determine if desired destination is local or remote

• Local or Remote Destination?– Upon determination of IP address

• IP host compares network portion of destination address to its own local network address

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Route Resolution Process (continued)

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If Remote, Which Router?

• Types of route table entries– Host route entry – Network route entry

• Receiving gateway typically does one of the following– Forwards packet– Sends an ICMP reply– Sends an ICMP reply indicating that it is unclear

where to send the packet

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Lifetime of an IP Datagram

• IP packets – Have a pre-defined lifetime indicated in each

packet’s Time to Live (TTL) field

• 64– Recommended starting TTL value

• 128– Default TTL in Windows 2000, Windows 2003, and

Windows XP

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Lifetime of an IP Datagram (continued)

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Fragmentation and Reassembly

• IP fragmentation – Enables a larger packet to be automatically

fragmented by a router

• Once fragmented– No reassembly occurs until fragments arrive at

destination– All fragments are given the same TTL value

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Service Delivery Options

• Precedence– Used by routers to determine what packet to send

• Type of Service– Used to select routing path when multiple paths exist– Routing protocols

• OSPF and Border Gateway Protocol (BGP)

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Differentiated Services and Explicit Congestion Notification

• RFC 2474, RFC 2475, and RFC 3168 – Offer a new use of the TOS field bits– Suggest that TOS and Precedence field bytes be

replaced by a Differentiated Services Code Point (DSCP) field

• Diffserv – Uses DSCP value to enable routers to offer varying

levels of service to traffic based on marker placed in the DSCP field

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IP Header Fields And Functions

• Version Field– First field in IP header

• Header Length Field– Denotes the length of the IP header only

• Type of Service Field– Has two components: precedence and Type of

Service• Total Length Field

– Defines length of the IP header and any valid data

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IP Header Fields And Functions (continued)

Guide to TCP/IP, Third Edtion 47

IP Header Fields And Functions (continued)

• Identification Field– Each packet is given a unique ID value when sent

• Flags Field– Three bits long– Typically, fragmentation is allowed

• Fragment Offset Field – Shows where to place packet’s data when

fragments are reassembled

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IP Header Fields And Functions (continued)

• Time to Live (TTL) Field– Denotes the remaining lifetime of the packet

• Protocol Field– Indicates what is coming up next

• Header Checksum Field– Provides error detection on the contents of the IP

header only

• Source Address Field– The IP address of the IP host that sent the packet

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IP Header Fields And Functions (continued)

• Destination Address Field– Can include a unicast, multicast, or broadcast

address– Final destination of the packet

• Options Fields– Exist primarily to provide additional IP routing

controls– Can be useful when testing or debugging code or

specific connections

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Summary

• Data link protocols – Manage transfer of datagrams across the network

• At Data Link layer– Protocols must deliver services, such as delimitation,

bit-level integrity checks, addressing, and protocol identification

• Ethernet II frames– Most common frame type on LANs

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Summary (continued)

• Understanding frame layouts – Crucial for proper handling of contents

• At the lowest level of detail– Important to understand the differences in field

layouts and meanings

• Imperative to understand how TCP/IP manages the translation between MAC layer addresses and numeric IP addresses

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Summary (continued)

• Proxy ARP – Permits router to interconnect multiple network

segments

• Network layer protocols – Make their way into the Data Link layer through a

process known as data encapsulation

• Important characteristics of IP datagrams– Time to Live (TTL) values– Fragmentation of incoming frames– Service delivery options